The endosperm of wheat, barley, rye and triticale contain large A-type and small B-type starch granules at maturity1. In wheat, the large A-type starch granules are more than 10 μm in diameter and lenticular in shape, while B-type starch granules are less than 10 μm in diameter and roughly spherical in shape2. Because A- and B-type starch granules have significantly different chemical compositions and functional properties3, wheat cultivars with predominantly A- or B-type starch granules would be very useful to the food and non-food industries.
A-type starch granules are produced in amyloplast at about four to five days-post-anthesis (DPA), and their number increases until 12 to 14 DPA4. Subsequently, the A-type starch granules grow in size to an eventual diameter of from 10 μm to more than 36 μm. The number of A-type starch granules per endosperm is constant from about 15 DPA to maturity.
B-type starch granules are actively initiated about 14–16 DPA. Both the number and size of B-type starch granules increase until wheat grain matures. The diameter of B-type starch granules is less than 10 μm2.The mechanisms controlling the initiation and size growth of A- and B-type starch granules are unknown. Based on the current knowledge about starch granule synthesis, several mechanisms could be proposed. The initiation and size growth of A- and B-type starch granules may be controlled by different isoforms of starch synthases (SS), starch branching enzymes (SBE), and debranching enzymes (DBE). These enzymes are involved in the biogenesis of plant starch granules5. In the barley shrunken endosperm mutant (shx), the size of A-type starch granules is reduced, giving the appearance of a unimodal size distributions. The soluble starch synthase I (SSS I) activity in the shx endosperm is 86% lower relative to the wild type, suggesting that SSS-I may play a role in controlling the size growth of A-type starch granules7. However, there are no experimental results showing genetic control of starch granule size distribution in wheat8, 9.
Starch branching enzyme (α-1,4-glucan-6-glycosyltransferase; EC 2.4.1.18, SBE) is a key enzyme in the starch biosynthesis pathway. The enzyme acts on glucose polymers and catalyses excision and transfer of glucan chains to the same or other glucan molecules. Translocated chains are attached to the polymer through α-1,6-glucosidic bonds to form branches on the α-1,4-linked glucose backbone. All of the reported SBE from plants to date can be divided into two classes, SBEI and SBEII, based on their amino acid sequences10. Most of the characterised plant SBEs are in the 80–100 kDa molecular mass range and, like all enzymes of the α-amylase family, carry a (βα)8 barrel domain with four highly conserved regions at the active site11. Analysis of plants with reduced SBEII activity and enzyme assays performed with purified SBEI and SBEII proteins suggest that the two enzyme classes differ in their enzymatic specificity12 13. The biochemical data suggest that SBEI favours transfer of long glucan chains and acts primarily on amylose, whereas SBEII produces shorter branches and prefers amylopectin as substrate14 15 16. However, the exact role of the different SBE classes in the formation of the branched glucan polymers in planta is not clear. There is no previous evidence to suggest that there are SBEs specific to A- or B-type starch granules.